1. Field of the Invention
This invention relates to reading information from magnetic cards and more specifically to a method for evaluating binary data stored in a magnetic track on a magnetic storage card.
2. Prior Art
Magnetic cards have found a wide range of application in the private and commercial sector, for instance as personal identification, as check-cards, as key-cards, etc. The information is stored on one or more tracks in a particular standardized code, such as the code described in International Standard (ISO) Ref. No. 7811/2-1985 (E), referred to as a two-frequency code. In a two-frequency code, flux changes, which are magnetic flux lines with alternating polarities, are imprinted on the track in fixed intervals. To indicate a binary value of 1, an additional flux change is inserted midway between two such fixed flux changes. Is this additional flux change absent, then the interval indicates the binary value 0. In order to read this magnetic information, the magnetic track is passed by an electromagnetic transformer with a more or less constant velocity, for instance by using a motor to move the magnetic card relative to a fixed transformer, or by passing a movable transformer over the stationary card, or by a person passing the magnetic card, by hand, lengthwise through a slot in which the transformer is mounted.
Due to this scanning movement, a voltage is induced in the transformer following the electrodynamic principle, the polarity of which depends on the polarity of the magnetic flux located on the magnetic track. Since the scanning velocity can vary greatly, the length of the time intervals between the flux changes, or rather the induced voltage impulses, must be determined by a calibration before the actual information can be read. For this purpose, according to the mentioned International Standard (ISO), flux changes are imprinted on the magnetic track in intervals indicating the binary value 0, on a sector which precedes the actual information.
In order to guarantee the safe reading of the stored information, the technical and physical properties of a magnetic card must generally lie within certain tolerance limits. This is the case with new and seldom used magnetic cards, but with increasing wear and age these properties of the magnetic card can change negatively. For instance, the card's surface form may be changed by mechanical and thermal influences, and may become bent or rippled. Signs of use, such as scratches, abrasion, dirt or hairline cracks may appear. Furthermore, the magnetic flux imprinted on the magnetic track may be weakened by superimposed interfering magnetic flux and through the influence of temperature. In addition to such changes in the magnetic card's properties, wear of the electromagnetic transformer, such as surface abrasion incorrect magnetization, as well as the presence of dirt, may inhibit the production of voltage impulses. These changes in the magnetic card's as well as the transformer's properties may cause the voltage impulses emitted by the transformer to be layered with interference signals which may either simulate binary information, or deform the curve of voltage impulses to such an extent that it lies outside of a given evaluation-range within which the signal could still be evaluated safely. As a result of this, the stored information may be read incorrectly or incompletely. The magnetic card at that point can no longer serve its intended function and has become worthless for its user.
In the commonly known process for reading a magnetic card, binary information is obtained through an analog evaluation of the voltage impulses. This evaluation is generally preceded by an amplification of the voltage impulses, which are then low-pass filtered and rectified. In order to recognize the maximum values of the rectified voltage impulses, which, as will be explained later, are used to determine the time intervals between impulses and thus gain the stored information, the time curve of the voltage impulses is differentiated and a discrimination circuit transforms the result into square wave impulses, the rising edges of which determine the time points of the maximum values. Through the use of these square wave impulses it becomes possible to register the voltage impulses at their maximum value and to evaluate the correlative time intervals between the voltage impulses in order to obtain the stored information.
However, such an evaluation will yield usable results only with magnetic cards which are in excellent condition. Should the voltage impulses be layered with interference signals, as described earlier, then the differentiation will produce interference impulses. The rising edges of the subsequently produced square wave signal will no longer define the extreme value of the voltage impulses, but rather the extreme value of the interference signal. Thus the time intervals between the voltage impulses will be incorrect, and the binary information stored on the magnetic track will be evaluated erroneously.
Thus it is an object of this invention to describe a process for the evaluation of binary information which will guarantee the reliable evaluation of the information stored magnetically on the magnetic track of a magnetic storage card, even if the magnetic track and/or the electromagnetic transformer is impaired by the effects of age, dirt, or other damage.